1. Field of the Invention
The present invention relates to an optical-level control apparatus for controlling the optical level of an optical signal to reduce the length of time it takes to recover the optical level to the original normal level in a switching operation and a recovery from a failure and a wavelength division multiplexing optical network comprising a plurality of nodes each employing the optical-level control apparatus.
2. Description of the Related Art
As the number of users utilizing the Internet and cellular phones increases, the amount of traffic also rises as well. In addition, services are also diversified from electronic business transactions and emails to distributions of moving pictures. On top of that, broadband communication becomes predominant and communication capacity also increases substantially. In consequence, a network having a large communication capacity is absolutely indispensable to communication. For this reason, introduction of an optical communication network is being carried forward. In particular, WDM (Wavelength Division Multiplexing) networks adopting a WDM technology are constructed at a very high pace. In addition, in consequence of increased communication capacities, contentions between carriers become severe and the cost of communication becomes higher considerably than the cost incurred so far. At the optical level, the technology of an optical node capable of handling wavelengths is a technology of importance. Examples of the optical node are an OADM (Optical Add Drop Multiplexing) node and an OXC (Optical Cross Connect) node.
As one of problems encountered in the conventional technology, in characteristics of optical components including an optical fiber and an optical amplifier, there are variations in optical power level among components of a wavelength division multiplexing signal, and these variations deteriorate the quality of transmission. In particular, if an optical node combining a variety of optical components is used, the variations become worse, requiring a function for adjusting the optical levels. In order to adjust the optical levels, an optical attenuator is introduced. Due to the operation of the optical attenuator, however, it takes time to restore the optical level of an output optical signal to the original normal level in a switching operation or in recovery from a failure.
FIG. 87 is a diagram showing a typical configuration of an optical node having a function for controlling levels of optical signals. A demultiplexer WDMUX 2#i demultiplexes an input WDM (wavelength division multiplexing) signal having wavelengths λ1 to λn into its optical components with the wavelengths λ1 to λn and supplies the optical components to a switch SW 4#i. If this optical node is an add node, the optical node has a transponder (TRPN) transmitter 6#i having a double configuration comprising a 0-system transmitter 6#i0 and a 1-system transmitter 6#il. In the transponder TRPN transmitter 6#i, one of the transmitters 6#i0 and 6#il is used for a work (W) system whereas the other transmitter is used for a protection (P) system. Typically, the 0-system transmitter 6#i0 is used for a work (W) system whereas the 1-system transmitter 6#il is used for a protection (P) system. Both the 0-system transmitter 6#i0 and the 1-system transmitter 6#il supply subscriber optical signals received from a subscriber to a switch SW 8#i. The switch SW 8#i selects an optical signal output by the work 0-system transmitter 6#0 and outputs the selected signal to a switch SW 4#i. It is to be noted that, if a failure occurs in the work 0-system transmitter 6#i0 of the TRPN transmitter 6#i, the operation is switched from the work 0-system transmitter 6#i to the protection 1-system transmitter 6#il. 
The switch SW 4#i passes on optical signals received from the demultiplexer 2#i and the switch SW 8#i as signals having different wavelengths to a transponder (TRPN) receiver 10#i or a first splitter SPL 12#ij where j=1, 2 and so on. If this optical node is a drop node, the transponder (TRPN) receiver 10#i receives the optical signals and passes on the signals to a subscriber.
The first splitter SPL 12#ij splits an input optical signal into two partial optical signals and supplies the partial optical signals to a variable optical attenuator VOA 14#ij and a first monitor PD 16#ij respectively. The output of the variable optical attenuator VOA 14#ij is connected to a second splitter SPL 18#ij. The second splitter SPL 18#ij further splits an optical signal output of the variable optical attenuator VOA 14#ij into two partial optical signals and supplies the partial optical signals to a multiplexer WMUX 20#i and a second monitor PD 22#ij respectively. The first monitor PD 16#ij and the second monitor PD 22#ij each detect an optical level and output an optical-level detection signal representing the detected optical level.
A control circuit 24#ij controls the variable optical attenuator VOA 14#ij to adjust the optical level of the signal output by the variable optical attenuator VOA 14#ij to a target level on the basis of the optical-level detection signals generated by the first monitor PD 16#ij and the second monitor PD 22#ij. The variable optical attenuator VOA 14#ij attenuates the optical signal received from the first splitter SPL 12#ij by applying an attenuation (ATT) quantity controlled by the control circuit 24#ij and outputs the attenuated optical signal to the SPL 18#ij. The multiplexer WMUX 20#i multiplexes optical signals having the wavelengths λ1to λ n and outputs a wavelength division multiplexing signal obtained as a result of the multiplexing to an optical fiber.
FIG. 88 is a diagram showing a transmission of an optical signal in a normal state. A node 30#1 is an add node, nodes 30#2 and 30#3 are each a thru node whereas a node 30#4 is a drop node. In a state where no failure occurs in the TRPN 0-system transmitter 6#10 employed in the node 30#1, as shown in FIG. 88, an added optical signal is supplied to the variable optical attenuator VOA 14#11 by way of the switch SW 4#1 and attenuated in the variable optical attenuator VOA 14#11 to a target level. By the same token, an optical signal output by the node 30#1 is supplied to the variable optical attenuator VOA 14#21 by way of the switch SW 4#2 and attenuated in the variable optical attenuator VOA 14#21 to a target level in the node 30#2. In the same way, an optical signal output by the node 30#2 is supplied to the variable optical attenuator VOA 14#31 by way of the switch SW 4#3 and attenuated in the variable optical attenuator VOA 14#31 to a target level in the node 30#3. Finally, an optical signal output by the node 30#3 is supplied to the TRPN receiver 10#4 by way of the switch SW 4#4 and dropped by the TRPN receiver 10#4 to a subscriber in the node 30#4.
FIG. 89 is a diagram showing a transmission of an optical signal in a state where a failure occurs. If a failure occurs in the TRPN 0-system transmitter 6#10 employed in the node 30#1 breaking an optical path including the TRPN 0-system transmitter 6#10 employed in the node 30#1, an optical beam transmitted to the node 30#2 to the node 30#4 disappears. Thus, the attenuation (ATT) quantities of the variable optical attenuator VOA 14#11, the variable optical attenuator VOA 14#21 and the variable optical attenuator VOA 14#31 become zero, which opens the variable optical attenuator VOA 14#11, the variable optical attenuator VOA 14#21 and the variable optical attenuator VOA 14#31 respectively. When the optical path is switched from the TRPN 0-system transmitter 6#10 to the TRPN 1-system transmitter 6#11 by the switch SW 8#1, the variable optical attenuator VOA 14#11, the variable optical attenuator VOA 14#21 and the variable optical attenuator VOA 14#31 have already been completely opened, causing an optical beam with a quantity greater than the normal one to enter the TRPN receiver 10#4 of the receiving node 30#4. In some cases, this optical beam damages the TRPN receiver 10#4. In addition, in the case of a WDM signal, the optical level of an optical signal transmitted to a channel involved in the failure becomes higher than the normal level at the recovery time so that, in some cases, it is quite within the bounds of possibility that this higher level has an effect on other channels.
In addition, if an optical loss of light failure of an optical signal causes the ATT quantity of a VOA to be increased, shutting down the VOA, no large difference in level between the normal condition and a failure-recovery state is resulted in. In this case, however, after the recovery from the failure, the VOAs need to adjust one signal level after another starting from the node 30#1. Documents describing technologies prior to the present invention include patent document 1, which discloses operations to increase the ATT quantity of a variable optical attenuator when a loss of light failure of an optical signal is detected and confirmed but decrease the ATT quantity of the variable optical attenuator when the input optical signal is restored to a normal state.
Nevertheless, the conventional technology has the following problems. Since one variable optical attenuator is activated after activation of the preceding one in a sequence of activations starting from node 1 as described above, the ATT quantity of the variable optical attenuator employed in the next node cannot be controlled correctly until the preceding node outputs an optical signal at a target level. Thus, the attenuation quantities of the variable optical attenuators employed in all nodes in the network cannot be controlled correctly either. The length of time it takes to correctly control the attenuation quantities of the variable optical attenuators employed in all nodes in the network is proportional to the number of nodes. In addition, it is difficult to operate the variable optical attenuator employed in each node in the network in a stable state and shorten the rise-time of the activation of each variable optical attenuator. This difficulty and the long rise-time of the activations raise another problem of an extremely long period of recovery time required in the event of a failure.
On top of that, if an optical signal disappears instantaneously on a temporary basis and recovered immediately in a state where optical signals are being communicated, in accordance with control of the ATT quantity of a variable optical attenuator to adjust the output quantity a fixed value, the ATT quantity of the variable optical attenuator is temporarily reduced in order to compensate for a drop in optical-signal level. When the worst comes to worst, however, the ATT quantity of the variable optical attenuator is excessively reduced to a minimum value. Then, when the loss of light state caused by the temporary loss of light failure disappears, the ATT quantity remains at a magnitude smaller than a normal value due to reasons including a delayed operation so that the optical level of an optical signal output by the variable optical attenuator rises, inducing an optical surge. It thus takes more time to restore the optical level of the optical signal output by the variable optical attenuator to the target level. Even if a function is provided for shutting down the variable optical attenuator in the event of a loss of light of optical signal, the optical level of the optical signal output by the variable optical attenuator must be restored to the target level in a recovery from a shut-down state (that is, a state of a maximum ATT quantity) in the same way. Thus, the conventional technology has a problem that it takes time to restore the optical level of the optical signal output by the variable optical attenuator to the target level from a shut-down state as is the case with the operation to recover from a failure as described above.
In addition, in accordance with patent document 1, control is executed to increase the ATT quantity in the event of a loss of light of an optical signal. Thus, the conventional technology disclosed in the document has a problem that it takes time to restore the optical level of an optical signal output by a variable optical attenuator to a normal level after the loss of light state disappears.